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Abstract:

This thesis is a study of freeplay - gaps between notionally connected components in
mechanical systems. For some systems any amount of freeplay can complicate closed
loop control design and introduce energetic impacts between bearing components. The
work was motivated by freeplay present in the control systems-of rotorcraft and explores the
viability of developing sensing devices to detect freeplay during operation.
In this work a generic freeplay model is introduced from first principles which predicts potential
measures of a system that could indicate the level freeplay. To validate these measures a mechanical
experiment is constructed which represents a minimum working example of a freeplay
system. It is designed to exclude as many sources of variability as possible and offers unparalleled
completeness and acuity of generated data. A novel mathematical model is derived (and found
to be in good agreement with the gathered experimental data) which validates the theorised
damage sensitive features.
Freeplay sensitive features are used as detection metrics in two real systems. First, on data
gathered from the experiment; A freeplay detection system was created which determined the
level of freeplay to a high accuracy using data gathered from a single accelerometer. Second, to
strain measurements taken from a rotorcraft in flight; It was found that some metrics derived
from earlier work were not good predictors of freeplay on rotorcraft data, but, using a selection of
data processing techniques, a bespoke freeplay detection system was successful for many flight
conditions.
The work in this thesis is a strong indicator that detection of freeplay is possible for certain
classes of systems using autonomous devices. Positive identification of freeplay in a simple and
a complex freeplay system have been shown and a critical review is offered of the conclusions,
limitations and some opportunities for future study.